course Phy 122 ՠgalassignment #006
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12:09:43 Query Principles and General Physics 17.4: work by field on proton from potential +135 V to potential -55 V.
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RESPONSE --> This exact problem is not in my book. It is very similar to 17.2, which reads: ""How much work does the electric field do in moving a proton from a point with a potential of +125V to a point where is is -55V? Express your answer both in joules and electron volts."" In this case, we set the change in potential energy equal to the work required of an external force: W = `dPE = q(Vb - Va) = (1.6*10^-19C)(-55V - 125V) = -2.88*10^-17 J The electron volts of the proton will be 180V*1e = 180eV.
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12:10:52 The change in potential is final potential - initial potential = -55 V - (125 V) = -180 V, so the change in the potential energy of the proton is {}{}-180 V * 1.6 * 10^-19 C ={}-180 J / C * 1.6 * 10^-19 C = -2.9 * 10^-17 J. {}{}In the absence of dissipative forces this is equal and opposite to the change in the KE of the proton; i.e., the proton would gain 2.9 * 10^-17 J of kinetic energy.{}{} Change in potential energy is equal and opposite to the work done by the field on the charge, so the field does 2.9 * 10^-17 J of work on the charge.{}{}Since the charge of the proton is equal in magnitude to that of an electron, he work in electron volts would be 180 volts * charge of 1 electron= 180 eV.
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RESPONSE --> I feel that I adequately answered this problem. However, I did not mention that in the absence of dissipative forces the work is equal and opposite to the change in kinetic energy.
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12:15:14 Query Principles and General Physics 17.8: Potential difference required to give He nucleus 65.0 keV of KE.
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RESPONSE --> We are told that the charge of the He nucleus is Q = 2e 65.0 keV = 6.5*10^4 eV If the nucleus receives 2e of KE for every unit of voltage, then to reach 65 kev the nucleus would need half of that value. 6.5*10^4 eV/ 2e = 3.25*10^4 V
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12:18:34 65.0 keV is 65.0 * 10^3 eV, or 6.50 * 10^4 eV, of energy. {}{}The charge on a He nucleus is +2 e, where e is the charge on an electron. So assuming no dissipative forces, for every volt of potential difference, the He nuclues would gain 2 eV of kinetic energy.{}{}To gain 6.50 * 10^4 eV of energy the voltage difference would therefore be half of 6.50 * 10^4 voles, or 3.35 * 10^4 volts.
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RESPONSE --> My answer was slightly different, 3.25*10^4 V.
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12:18:44 Query gen phy text problem 17.18 potential 2.5 * 10^-15 m from proton; PE of two protons at this separation in a nucleus. What is the electrostatic potential at a distance of 2.5 * 10^-15 meters?
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RESPONSE --> I am not a general physics student, but I would like to try to answer this question. a)Electric potential can be found by the equation V = kQ/r, where Q would be the charge of the proton (same magnitude as an electron) and r is the distance to the proton, 2.5*10^-15m. V = [(9*10^9 Nm^2/C^2)(1.6*10^-19C)]/(2.5*10^-15m) = 5.76*10^5 V b) I don't know how to complete this part.
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12:19:18 STUDENT SOLUTION: For a part, to determine the electric potential a distance fo 2.5810^-15m away from a proton, I simply used the equation V = k q / r for electric potential for point charge: q = 1.60*10^-19C=charge on proton V = kq/r = 9.0*10^9N*m^2/C^2(1.60*10^-19C) / (2.5*10^-15m) = 5.8*10^5V. Part B was the more difficult portion of the problem. You have to consider a system that consists of two protons 2.5*10^-5m apart. The work done against the electric field to assemble these charges is W = qV. The potential energy is equal to the work done against the field. PE=(1.60*10^-19C)(5.8*10^5V) = 9.2*10^-14 J.
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RESPONSE --> I have read the solution, but I don't quite understand what this student did in part B. I know that this question wasn't meant for my section, but I would like to know how to do it.
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12:19:36 query univ phy 23.58 (24.58 10th edition). Geiger counter: long central wire 145 microns radius, hollow cylinder radius 1.8 cm. What potential difference between the wire in the cylinder will produce an electric field of 20,000 volts/m at 1.2 cm from the wire?
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RESPONSE --> I am not a university of physics student
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12:19:49 ** The voltage V_ab is obtained by integrating the electric field from the radius of the central wire to the outer radius. From this we determine that E = Vab / ln(b/a) * 1/r, where a is the inner radius and b the outer radius. If E = 20,000 V/m at r = 1.2 cm then Vab = E * r * ln(b/a) = 20,000 V/m * ln(1.8 cm / .0145 cm) * .012 m = 1157 V. **
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RESPONSE --> ok
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12:20:01 Query univ 23.78 (24.72 10th edition). Rain drop radius .65 mm charge -1.2 pC. What is the potential at the surface of the rain drop?
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RESPONSE --> I am not a university of physics student
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12:20:40 STUDENT RESPONSE FOLLOWED BY SOLUTION: The problem said that V was 0 at d = inifinity, which I understnad to mean that as we approach the raindrop from infinity, the potential differencegrows from 0, to some amount at the surface of the raindrop. Because water molecules are more positive on one side that the other, they tend to align in a certain direction. Since positive charges tend to drift toward negative charge, I would think that the raindrop, with its overall negative charge, has molecules arranged so that their more positive sides are pointing toward the center and negative sides will be alighed along the surface of the raindrop. Probably all wrong. I tried several differnet integrand configuraitons but never found one that gave me an answer in volts. SOLUTION: You will have charge Q = -1.2 * 10^-12 C on the surface of a sphere of radius .00065 m. The field is therefore E = k Q / r^2 = 9 * 10^9 N m^2 / C^2 * (-1.2 * 10^-12 C) / r^2 = -1.08 * 10^-2 N m^2 / C / r^2. Integrating the field from infinity to .00065 m we get (-1.08 * 10^-2 N m^2 / C) / (.00065 m) = -16.6 N m / C = -16.6 V. If two such drops merge they form a sphere with twice the volume and hence 2^(1/3) times the radius, and twice the charge. The surface potential is proportional to charge and inversely proportional to volume. So the surface potential will be 2 / 2^(1/3) = 2^(2/3) times as great as before. The surface potential is therefore 16.6 V * 2^(2/3) = -26.4 volts, approx.. **
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RESPONSE --> ok
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